Traditional liquid crystal displays (LCDs) are built using two glass substrates with conductive but optically transparent indium-tin-oxide electrodes. The electrodes are typically covered with an alignment layer, which anchors the liquid crystal (LC) molecules along a predetermined direction to obtain a specific liquid crystal director (or optic axis) configuration. The optic axis is then manipulated with the help of an applied electric field to change the cell's birefringence and to create different optical transmission states. Flexible LCDs made of plastic substrates have many advantages such as their light weight, small thickness, and flexibility. The biggest challenge related to the construction of flexible devices are spacing (cell gap) control and resistance to mechanical stress.
Traditionally, glass or plastic beads or fibers have been used to maintain uniform cell gap in LCDs that are made with glass substrates. But in LC cells built with flexible plastic substrates, the spacers tend to move around and aggregate, especially when the cell gap is temporarily increased by mechanical deformations, thus causing a loss of spacing control. A change in the cell gap invariably has an adverse impact on the cell's optical characteristics.
Flexible LCDs using plastic substrates can be built using the polymer dispersed LC (PDLC) technology developed by J. Doane et al. in the mid 1980's. However PDLCs have never been used in commercial display products because of their high driving voltage and low multiplexibility.
A method of making an LCD known as the polymer walls method, relies on formation of pure polymer regions in between the pixels. This is accomplished by the application of a “patterned” electric field to cause phase separation of a prepolymer-liquid crystal solution, followed by UV-irradiation to polymerize the segregated polymer regions. The polymer walls provide spacing control and pressure resistance preventing distortion of the displayed image. However, the formation of polymer walls requires patterned electrodes and a high electric field (˜18V/μm). This has prevented the technology from being applied in commercial LCDs. Furthermore, it has not been possible to successfully create polymer walls in devices with rubbed alignment layers coated over ITO electrodes.
A LC cell prepared by the phase separated composite film (PSCOF) technology, has a bi-layer structure due to complete anisotropic phase separation of a solution of LC and prepolymer. U.S. Pat. No 5,949,508, the disclosure of which is hereby incorporated by reference, discloses a phase separated composite film. A layer of polymer is formed on the substrate close to the UV source used to initiate phase separation. The LC is expelled from the polymer film to form a uniform layer near the surface of the second substrate. The LC's optic axis is aligned along the rubbing direction, as in conventional devices. Some small areas of the polymer film extend out and bond to the opposite substrate. These bonding sites provide good resistance to mechanical pressure and render it possible to build flexible LCDs on plastic films. However, in such devices only one substrate has an alignment layer to provide anchoring and a pretilt angle.
There have also been other proposed structures for LC devices internally containing liquid crystal and polymer. For example, International Patent Application No. PCT/US00/03866 provides a phase separated composite organic structure which can function in light modulating, beam steering, and focusing elements. The method provides a polymeric material predominantly adjacent to one of the two substrates of the cell and a defined microstructure of low molecular weight organic material primarily adjacent to the other substrate.
Another LC device internally containing polymer and LC is disclosed in U.S. Pat. No. 5,680,189. This patent provides a resin material that is dispersed in a liquid crystal material. The resinous columns are precipitated or deposited out of the liquid crystal to form columns within the cell. However, the use of phase separation of a resin from a liquid crystal in which it is miscible at a given temperature is not provided.
U.S. Pat. No. 5,739,882 provides a LCD that contains polymerized column spacers. In that patent, the column spacers are formed on a modified substrate or otherwise are located in a non-random manner. The columns are also formed from an ultraviolet-curable or heat-curable resin. As with the '189 patent described above, the resin columns are formed by precipitation and not by phase separation, because phase separated resin would have re-dissolved into a liquid crystal upon heating. Additionally, the location of the resin columns may be controlled by modifying the surface of at least one of the substrates.
Electrooptical displays containing internal spacers are also provided by U.S. patent application Publication No. 2002/0109807. In that Patent Application, a parameter that effects either the rate of polymerization of the polymer or the rate of phase separation is adjusted so as to form a first and second layer within the a liquid crystal device. This application also provides a liquid crystal device containing the internal spacers that are coated with a polymer initiation or enhancement (PIE) material. The composition of the LC and the pre-polymer are chosen so that polymer forms around the spacers and is localized to those areas. A liquid crystal cell also may include non-structural polymer initiation or enhancement material. In such a cell, such as one including a mesh-like PIE, the polymer localizes around the PIE and may also extend to one substrate or the other, forming isolated areas of liquid crystal.
In general, it has been difficult to fabricate low cost, highly multiplexed, and flexible displays using the existing methods. Therefore, there is a need for an alternate method of producing a flexible LCD.